This invention relates in general to the manufacture of ceramic hybrid microcircuits and more particularly to a novel material system for making thick film resistors on a ceramic substrate.
Present methods utilized in the manufacture of the thick film resistors include a multi-stepped process which builds the resistors and interconnects on the substrate. This process first includes printing, drying and firing of a conductor material, normally palladium-silver (Pd-Ag), as pads, interconnects and terminations. Then, a Ruthenium based resistor material is printed on the substrate between the palladium-silver pads. The deposited resistor material is subsequently dried and fired. Next, a glass encapsulant is printed, dried and fired over the conductor pads and resistor. Finally, the the thick film resistor is laser trimmed to tolerance.
The major disadvantage of the process outlined above is material cost. Palladium-silver paste is generally expensive. Further, since it is a precious metal its cost is subject to wild and rapid market fluctuations. This price cost fluctuation provides difficulty in pricing circuits and budgeting for manufacturing cost.
The thick film industry has been searching for an alternative to precious metal conductors and as a result has developed base metal conductors, like copper, which can provide conductors with greater conductivity then with palladium-silver material. However, base metal conductors must be fired in a nitrogen atmosphere. Unfortunately, resistor paste technology still required the use of air firing.
In order to make the conductor firing compatible with the presently known resistor pastes, material manufactures developed low temperature firing copper conductors which can be used with air fired resistor technology. Air fired resistors compatible with the copper conductor material are not compatible with any nitrogen fired glass overcoats. Air fired overcoats cannot be used since air firing will result in oxidation of the copper film.
Encapsulants are required to provide long term stability to the thick film resistors of less than 0.25% ohms, per 1000 hours, at 150 degrees C. to 85 degrees C.
It therefor becomes an object of the present invention to provide a novel thick film material system for making thick film resistors using base metal conductors and encapsulation.